1. Field of the Invention (Technical Field)
This present invention relates to night vision imaging systems (NVIS), and more particularly to backlights for transmissive displays required to meet NVIS radiance requirements. It also relates to general illumination assemblies, which are used with night vision goggles.
2. Background Art
The present invention resulted from a desire to lower the cost of NVIS compliant backlights while maintaining optical efficiency. Typically optical filters used to achieve NVIS compliance for full color displays are constructed from two distinct optical components, a reflective component and an absorptive component. The absorptive component typically represents more than half of the cost of the NVIS filter and it attenuates the visible spectrum by more than half of the total loss associated with the entire NVIS filter.
Presently NVIS compliance is achieved by the addition of an optical filter between the display's light source and the user. This filter may be over the entire backlight opening or just over the light source used for NVIS mode operation. There are multiple examples of these types of backlight architectures.
Honeywell's U.S. Pat. No. 5,661,578, entitled A Color Liquid Crystal Display Backlight System for Night Vision Imaging System Compatibility, uses a NVIS filter located over the nighttime lamp assembly used to achieve NVIS compliance. U.S. Pat. No. 6,111,622, entitled A day/night backlight for a liquid crystal display describes the use of a sharp cut-off IR filter and an IR absorptive filter to achieve NVIS compliance. This patent also describes placing the entire filter adjacent to the LCD stack. U.S. Pat. No. 6,100,952, entitled A NVG-compatible AMLCD backlight having a ridged prismatic TIR with an embedded diffuser, describes the use of an infrared reflecting filter and a diffuser with an infrared absorbing die to achieve NVIS compliance. Both of these elements are placed adjacent to the AMLCD between the light source and the display user. U.S. Pat. No. 6,842,204 uses a NVIS filter over the NVIS mode light sources, which is also situated between the light source and the display user.
These existing prior art approaches for providing NVIS compliance for a full color display make use of a NVIS filter that contains infrared absorbing materials, either glass or plastic filters. The infrared absorptive materials are typically expensive optical elements, which are in the direct optical path of the output illumination thereby decreasing the backlight system's efficiency by attenuating visible illumination.